Continuous silica production process and silica product prepared from same

ABSTRACT

Disclosed herein is a continuous process for preparing a silica product, comprising: (a) continuously feeding an acidulating agent and an alkali metal silicate into a loop reaction zone comprising a stream of liquid medium; wherein at least a portion of the acidulating agent and the alkali metal silicate react to form a silica product in the liquid medium of the loop reaction zone; (b) continuously recirculating the liquid medium through the loop reaction zone; and (c) continuously discharging from the loop reaction zone a portion of the liquid medium comprising the silica product. Silica products and dentifrice compositions comprising the silica products are also disclosed. A continuous loop reactor is also disclosed.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 14/557,498, filed on Dec. 2, 2014, now U.S. Pat.No. 9,327,988, which is a divisional application of co-pending U.S.patent application Ser. No. 14/078,589, filed on Nov. 13, 2013, now U.S.Pat. No. 8,945,517, which is a continuation application of U.S. patentapplication Ser. No. 12/711,321, filed on Feb. 24, 2010, now U.S. Pat.No. 8,609,068, the disclosures of which are incorporated herein byreference in their entirety.

BACKGROUND

Precipitated silica can be prepared by adding an acidulating agent to analkali metal silicate to precipitate amorphous silica. The resultingprecipitate is usually filtered away from the reaction medium andsubsequently washed and dried. Typically, the dried silica is thenmechanically comminuted in order to provide a suitable particle size andsize distribution. On an industrial scale, silica can be prepared by astep-wise batch process that incorporates the aforementioned steps. Theequipment needed for such a process can be capital intensive and oftenleads to inefficiency in the process, particularly when idle time existswhen reactants are not being consumed. While various other silicaproduction processes exist, many of these processes are difficult tocontrol and scale-up, and many still require extensive processing stepsafter the silica has been prepared.

A need therefore exists for improved silica production processes thataddress the aforementioned shortcomings in traditional silica productionprocesses. This need and other needs are satisfied by the presentinvention.

SUMMARY

Disclosed herein is a continuous process for preparing a silica product,comprising: (a) continuously feeding an acidulating agent and an alkalimetal silicate into a loop reaction zone comprising a stream of liquidmedium; wherein at least a portion of the acidulating agent and thealkali metal silicate react to form a silica product in the liquidmedium of the loop reaction zone; (b) continuously recirculating theliquid medium through the loop reaction zone; and (c) continuouslydischarging from the loop reaction zone a portion of the liquid mediumcomprising the silica product.

Also disclosed are silica particles having an oil absorption value of upto 100 cc/100 g; wherein at least 80% of the silica particles arerounded to well rounded; and wherein the silica particles have asphericity (Sao) factor of greater than 0.9 and a Brass EinlehnerAbrasion value of less than 8.0 mg lost/100,000 revolutions.

Also disclosed are silica particles having a particle size of from 3 to15 μm, an oil absorption value of greater than 100 cc/100 g, and aPellicle Cleaning Ratio (PCR) value at 20% silica loading of at least85.

Also disclosed are dentifrice compositions comprising silica particlesin an amount ranging from 5 to 50% by weight of the composition; whereinthe silica particles have an oil absorption value of up to 100 cc/100 g,a sphericity (S₈₀) factor of greater than 0.9, and a Brass EinlehnerAbrasion value of less than 8.0 mg lost/100,000 revolutions; wherein atleast 80% of the silica particles are rounded to well rounded.

Also disclosed are dentifrice compositions comprising silica particlesin an amount ranging from 5 to 50% by weight of the composition; whereinthe silica particles have a particle size of from 3 to 15 μm, an oilabsorption value of greater than 100 cc/100 g, and a Pellicle CleaningRatio (PCR) value at 20% silica loading of at least 85.

The advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the aspects describedbelow. The advantages described below will be realized and attained bymeans of the elements and combinations particularly pointed out in theappended claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary continuous loop reactor.

FIG. 2 is a plot showing Horiba particle size scans for Example 2E inslurry (circle), spray dried (diamond) and hammer milled (triangle).ZEODENT 103 silica is shown for comparison (square).

FIGS. 3A and 3B are Scanning Electron Micrographs (SEM) of Example 2Dprepared by the disclosed process.

FIGS. 4A and 4B are SEM images of Example 2R prepared by the disclosedprocess.

FIGS. 5A and 5B are SEM images of Example 2E prepared by the disclosedprocess.

FIGS. 6A and 6B are SEM images of ZEODENT 113 and ZEODENT 165.

FIG. 7 is a SEM image of Example 2F prepared by the disclosed process.

FIG. 8 is a graphical representation of particle roundness.

FIG. 9 is a pictorial representation for index of roundnesscalculations.

DETAILED DESCRIPTION

Before the present compounds, compositions, composites, articles,devices and/or methods are disclosed and described, it is to beunderstood that the aspects described below are not limited to specificcompounds, compositions, composites, articles, devices, methods, or usesas such may, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting.

In this specification and in the claims that follow, reference will bemade to a number of terms that shall be defined to have the followingmeanings:

Throughout this specification, unless the context requires otherwise,the word “comprise,” or variations such as “comprises” or “comprising,”will be understood to imply the inclusion of a stated integer or step orgroup of integers or steps but not the exclusion of any other integer orstep or group of integers or steps.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “an acidulating agent” includes mixtures of two or moresuch agents, and the like.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

Ranges may be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Disclosed are compounds, compositions, and components that can be usedfor, can be used in conjunction with, can be used in preparation for, orare products of the disclosed methods and compositions. These and othermaterials are disclosed herein, and it is understood that whencombinations, subsets, interactions, groups, etc. of these materials aredisclosed that while specific reference of each various individual andcollective combinations and permutation of these compounds may not beexplicitly disclosed, each is specifically contemplated and describedherein. For example, if a number of different acidulating agents andalkali metal silicates are disclosed and discussed, each and everycombination and permutation of the acidulating agent and metal silicateare specifically contemplated unless specifically indicated to thecontrary. Thus, if a class of agents A, B, and C are disclosed as wellas a class of agents D, E, and F and an example of a combination ofagents, A-D is disclosed, then even if each is not individually recited,each is individually and collectively contemplated. Thus, in thisexample, each of the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, andC-F are specifically contemplated and should be considered disclosedfrom disclosure of A, B, and C; D, E, and F; and the example combinationA-D. Likewise, any subset or combination of these is also specificallycontemplated and disclosed. Thus, for example, the sub-group of A-E,B-F, and C-E are specifically contemplated and should be considereddisclosed from disclosure of A, B, and C; D, E, and F; and the examplecombination A-D. This concept applies to all aspects of this disclosureincluding, but not limited to, steps in methods of making and using thedisclosed compositions. Thus, if there are a variety of additional stepsthat can be performed it is understood that each of these additionalsteps can be performed with any specific embodiment or combination ofembodiments of the disclosed methods, and that each such combination isspecifically contemplated and should be considered disclosed.

Process for Preparing Silica Product

In one aspect, the process of the invention is a continuous processwherein an acidulating agent and an alkali metal silicate arecontinuously fed into a loop reaction zone comprising a stream of liquidmedium; wherein at least a portion of the acidulating agent and thealkali metal silicate react to form a silica product in the liquidmedium of the loop reaction zone. As the acidulating agent and alkalimetal silicate are continuously fed into the loop reaction zone, thecontents of the loop reaction zone (i.e., the liquid medium) arecontinuously recirculated. Silica product is collected by discharging aportion of the liquid medium that contains the silica product, which inone aspect is equal to the volume of raw materials added to the into theloop reaction zone.

As used herein, the “loop reaction zone” refers to an area inside areactor that forms a continuous circuit that contains the recirculatingliquid medium wherein the acidulating agent and the alkali metalsilicate react to form the silica product. As will be discussed below,in one aspect, the loop reaction zone is defined by walls of acontinuous loop of one or more loop reactor pipes. Generally, the liquidmedium in the loop reaction zone will vary in composition depending onthe stage of the process. Prior to adding the acidulating agent andalkali metal silicate into the liquid medium, the medium can containonly water or a suitable aqueous solution or dispersion (slurry). In oneaspect, prior to feeding the acidulating agent and the alkali metalsilicate into the reaction zone, the liquid medium can contain seedsilica, which can serve to reduce gelation in the loop reaction zone andassist in forming the silica product. In a specific aspect, prior toadding acidulating agent and alkali metal silicate, precipitated silica,sodium sulfate, sodium silicate, and water can first be added to theloop reaction zone and recirculated as desired, after which acidulatingagent and alkali metal silicate can be added. As acidulating agent andalkali metal silicate are fed into the loop reaction zone, silicaproduct forms in the liquid reaction medium. The silica product willgenerally be a precipitated product, and thus will be a dispersed phasein the liquid reaction medium. In one aspect, prior to collectingdesired silica product, the seed silica product can be purged from theloop reaction zone.

The process temperature and pressure can also vary widely and can dependon what type of silica product is desired. In one aspect of the process,a temperature of from about ambient temperature to about 130° C. ismaintained in the liquid medium. Likewise, a variety of pressures can beused. The pressure can range from atmospheric pressure to higherpressures. For example, when a continuous loop reactor is used with theprocess, the reactor can be fitted with a back-pressure valve forcontrolling a wide range of pressures inside the reactor.

The alkali metal silicate and acidulating agent can be fed into thereaction zone at various rates. The rate of addition of the alkali metalsilicate is generally such that a desired concentration of silicate ismaintained in the reaction zone, whereas the rate of addition of theacidulating agent is such that a desired pH is maintained in the loopreaction zone. In one aspect, the alkali metal silicate is fed into theloop reaction zone at a rate of at least 0.5 L/min. The maximum alkalimetal silicate addition rate will vary widely depending on the volume ofthe loop reaction zone and scale of the silica production process. Ahigh silicate addition rate could be desired, for example, in a verylarge scale process wherein a large volume of reactants are being used.In one specific example, the alkali metal silicate is fed at a rate offrom 0.5 to 5 L/min, or from 0.5 to 3 L/min.

The acidulating agent is generally fed into the loop reaction zone at arate sufficient to maintain a pH of from 2.5 to 10.5 in the liquidmedium. In other aspects, the acidulating agent is fed into the loopreaction zone at a rate sufficient to maintain a pH of from 7.0 to 10 inthe liquid medium, or from 7.0 to 8.5 in the liquid medium. For example,in a specific aspect, a pH of about 7.5 is maintained in the liquidmedium. The pH of the liquid medium can be monitored by any conventionalpH sensitive electrode. In some examples, the pH of the liquid mediumcan be evaluated by directly measuring the pH of the liquid medium(slurry). In these examples, the pH of the liquid reaction medium willgenerally range from 2.5 to 10.5, from 6 to 10, or from 7 to 8.5.

The liquid medium can be recirculated at various rates, depending on theconditions present in the loop reaction zone, such as degree of mixingor shear present in the reaction zone, and depending on the scale of theproduction process. Generally, the liquid medium is recirculated throughthe loop reaction zone at a rate of at least 15 L/min. In a specificexample, the liquid medium can be recirculated through the loop reactionzone at a rate of from 15 to 100 L/min, from 30 to 80 L/min, or from 70to 80 L/min.

A variety of acidulating agents can be used, including various acids andother agents capable of reacting with the alkali metal silicate to formthe silica product. The acid, or acidulating agent, can be a Lewis acidor Brönsted acid, such as a strong mineral acid, for example, sulfuricacid, hydrochloric acid, nitric acid, phosphoric acid, and so forth.Such acids can be added into the reaction zone as dilute solutions. As aspecific example, a 6 to 35% by weight, and more preferably a 10 to 17%by weight solution of sulfuric acid as acidulating agent can be fed intothe loop reaction zone. In other aspects, a gas such as CO₂, can be usedas the acidulating agent. Carbon dioxide produces a weak acid (carbonicacid), and thus it can be desirable for the liquid medium to be kept ata pH target of greater than about 8.5 when such a weak acid is used.

In a further aspect, the acidulating agent can be selected based on thetype of silica product that is desired. For example, an acidic solutionof aluminum sulfate can be used as the acidulating agent, and theresulting silica product will therefore be an alkali aluminosilicate. Asa specific example, aluminum sulfate can be added to sulfuric acid, andthis mixture can be used as the acidulating agent.

Any suitable alkali metal silicate can be used with the process of theinvention, including both metal silicates, disilicates, and the like.Water soluble potassium silicates and sodium silicates are particularlypreferred. In general, acceptable silica products of this invention canbe made with silicates having various alkali metal:silicate molarratios. For a sodium silicate, for example, the molar ratio, Na₂O:SiO₂,will generally range from about 1:1 to 1:3.5 and preferably from about1:2.4 to about 1:3.4. The alkali metal silicate fed into the loopreaction zone is preferably fed as an aqueous solution, similar to theacidulating agent. The alkali silicate solution fed into the loopreaction zone generally can contain between about 8 to 35%, and morepreferably between about 8% and 20% by weight alkali metal silicatebased on the total weight of the alkali metal silicate solution fed intothe loop reaction zone.

When desired, and in order to reduce the alkali silicate or acidulatingagent concentration of a source solution, dilution water can be added tothe source solution before the solution is fed into the loop reactionzone, and/or the dilution water can be added separately into the loopreaction zone and subsequently mixed with the alkali metal silicateand/or acidulating agent and any other liquid medium contents.

As the desired quantity of acidulating agent and alkali metal silicateare added into the loop reaction zone, the liquid medium will generallybe recirculated on average a minimum of three passes through therecirculation zone. The number of times the liquid medium isrecirculated through the loop reaction zone, on average, is referred toherein as the “mean number of passes,” which is calculated according tothe following equations. The residence time of the silica product in therecirculation loop before discharge is calculated by dividing thereaction system volume by the raw material addition rate (alkali metalsilicate addition rate+acidulating agent addition rate). The number ofpasses/minute can then be calculated by dividing the recirculation rateby the total system volume. The residence time can then be multiplied bythe number of passes/minute to get the mean number of passes.

${{residence}\mspace{14mu}{{time}\left( \min \right)}} = \frac{{system}\mspace{14mu}{{volume}(L)}}{{combined}\mspace{14mu}{raw}\mspace{14mu}{material}\mspace{14mu}{addition}{\mspace{11mu}\;}{{rate}\left( {L\text{/}\min} \right)}}$${{number}\mspace{14mu}{of}\mspace{14mu}{passes}\text{/}\min} = \frac{{recirculation}\mspace{14mu}{{rate}\left( {L\text{/}\min} \right)}}{{system}{\mspace{11mu}\;}{{volume}(L)}}$${{residence}{\mspace{11mu}\;}{{time}\left( \min \right)} \times \frac{{number}{\mspace{11mu}\;}{of}\mspace{14mu}{passes}}{\left( \min \right)}} = {{mean}\mspace{14mu}{number}\mspace{20mu}{passes}}$

The silica product can be recirculated such that the mean number ofpasses is from 3 to 200, or from 10 to 200. Generally, the higher themean number of passes, the more spherical and rounded the silica productbecomes. The number of recirculation passes (mean number of passes) cantherefore be selected based on the type of silica product that isdesired.

Silica product can be discharged from the loop reaction through variousmechanisms. In one aspect, a continuous loop reactor is used in theprocess, as discussed below, which can contain a valve for releasing thesilica product from the loop reaction zone. Preferably, however, silicaproduct is displaced from the loop reaction zone by adding additionalliquid into the reaction zone such that a portion of the liquid mediumcontaining the silica product is discharged from the reaction zone(i.e., the reaction zone is overflowed). This can be accomplished in oneaspect by continuously adding acidulating agent and/or alkali metalsilicate into the loop reaction zone as a portion of the liquid mediumis volumetrically displaced by the volume of acidulating agent and/oralkali metal silicate that is being added.

In some aspects of the process, the acidulating agent and alkali metalsilicate are continuously added while the liquid reaction medium isbeing recirculated and while silica product is being discharged. Thus,in one aspect, each step of the process occurs continuously andsimultaneously. In a further aspect, the acidulating agent and alkalimetal silicate are each fed into the loop reaction zone simultaneously.The acidulating agent and alkali metal silicate are preferably added tothe loop reaction at different points along the loop reaction zone. Forexample, alkali metal silicate can be added upstream in the looprelative to the acidulating agent, such that as the acidulating agent isbeing fed into the reaction zone, alkali metal silicate is alreadypresent.

Modifications to the structure of the silica product can be achieved bymodifications to temperature, ionic strength, addition rates, and energyinput. Generally, changes in temperature, recirculation rate, andacidulating agent/alkali metal silicate addition rates result in thelargest changes to the physical properties of the silica products.Generally, the more the liquid medium is recirculated, the longer theresidence time of the silica product in the recirculation loop (sloweraddition rates), and the higher the temperature, the lower the structure(as defined by oil absorption) of the resulting silica product.Manipulations to the pH in the liquid medium were observed to minimizesilica deposits (fouling) within the loop reaction zone when a pH ofbelow about 9.0 was used.

The silica product can be collected after being discharged from the loopreaction zone in a suitable vessel and processed as desired. In someaspects, the silica product requires no further processing (other thanwashing to remove salts, etc.) and can be shipped as a wet cake or canbe dried as desired. In one aspect, for example, the resulting silicaproduct can be spray dried according the methods known in the art.Alternatively, a wet cake of the silica product can be obtained and canbe reslurried and handled and supplied in slurry form or supplied as afilter cake, directly. Generally, drying of the silica product describedherein can be effected by any conventional equipment used for dryingsilica, e.g., spray drying, nozzle drying (e.g., tower or fountain),flash drying, rotary wheel drying or oven/fluid bed drying. The driedsilica product generally should have a 1 to 15 wt. % moisture level. Thenature of the silica reaction product and the drying process both areknown to affect the bulk density and liquid carrying capacity.

In other aspects, the silica product can be subjected to varioustreatments, depending on the nature of the desired silica product. Forexample, after the silica product is collected, the pH of the silicaslurry can be adjusted, e.g., lowered using an acid such as sulfuricacid, following by filtering and washing. In this example, the silicaproduct can be washed to a desired conductivity, for example, from 1500μS to 2000 μS, followed by drying as discussed above.

To decrease the size of the dried silica product further, if desired,conventional grinding and milling equipment can be used. A hammer orpendulum mill may be used in one or multiple passes for comminuting andfine grinding can be performed by fluid energy or air-jet mill. Productsground to the desired size may be separated from other sizes byconventional separation techniques, e.g., cyclones, classifiers orvibrating screens of appropriate mesh sizing, and so forth.

There are also ways to reduce the particle size of the resulting silicaproduct before isolation and/or during the synthesis of the silicaproduct that affect the size of the dried product or product in slurryform. These include but are not limited to media milling, the use ofhigh shear equipment (e.g., high shear pump or rotor-stator mixers), orultrasound devices, which in some aspects, can be used during theproduction process itself, for example in the recirculation loop.Particle size reduction carried out on the wet silica product can bedone at anytime before drying.

Silica Product

A variety of types of silica product can be prepared using the disclosedprocess, depending on the starting materials and process conditions. Inone aspect, the silica products of the invention are silica particleshaving an oil absorption value of up to 100 cc/100 g. In this aspect, atleast 80% of the silica particles are rounded to well rounded. Thesesilica particles also have a sphericity (S₈₀) factor of greater than 0.9and a Brass Einlehner Abrasion value of less than 8.0 mg lost/100,000revolutions.

As used herein “rounded” particles are those having gently roundedcorners with flat faces and small reentrants nearly absent. “Wellrounded” particles are those having a uniform convex grain outline withno flat faces, corners, or reentrants discernable.

Characterization of the silica particles of the invention as rounded towell rounded is carried out according to the following procedure. Arepresentative sample of silica particles is collected and examined byscanning electron microscopy (SEM). Pictures are taken at two differentmagnification levels that are representative of the entire image. Thefirst image is taken at a magnification of approximately 200 times andis used to get a sense of the sample homogeneity. Next, an SEM imagewith a magnification of approximately 20,000 is evaluated. Preferably,there should be a minimum of approximately 20 particles that are shownin the image and care should be taken to insure the picture isrepresentative of the sample as a whole. The particles in this image arethen evaluated and characterized by class according to Table 1. At least80% of the particles of the invention that have oil absorption values upto 100 cc/100 g can be characterized as rounded to well rounded.

TABLE 1 Particle roundness characterization Class Description AngularStrongly developed faces with sharp corners. Sharply defined, largereentrants with numerous small reentrants. Subangular Strongly developedflat faces with incipient rounding of corners. Small reentrants subduedand large reentrants preserved. Subrounded Poorly developed flat faceswith corners well rounded. Few small and gently rounded reentrants, andlarge reentrants weakly defined. Rounded Flat faces nearly absent withcorners all gently rounded. Small reentrants absent. Well No flat faces,corners or reentrants discernible, and Rounded a uniform convex grainoutline.

To assist in characterizing particle roundness, the standard silhouettesgraph shown in FIG. 8 can be used. The particles as displayed in themagnified SEM image are compared to the standard particle roundnesschart shown in FIG. 8 and classified accordingly. This process iscommonly carried out in sedimentation science. As a specific example,the particles shown in FIGS. 3-5, which were prepared by the disclosedprocess, were classified by comparison to FIG. 8 as rounded towell-rounded in nature, meaning that at least 80% of the particles arerounded to well rounded. In contrast, the silica products shown in FIG.6, which were prepared by a traditional batch processes, were classifiedby comparison to FIG. 8 as predominantly angular, sub-angular andsub-rounded, since flat sides and sharp, jagged edges can be observed.

The silica particles of the invention that have oil absorption values ofless than 100 cc/100 g can also be characterized according to an indexof roundness. As used herein “index of roundness” is defined as theratio of the radii of curvature of the corners and edges and the radiusof the maximum inscribed circle of the particle. Index of roundness canbe calculated according to the following equation:

${{{Roundness}\mspace{14mu}{Index}} = \frac{\left( {\sum r} \right)\text{/}N}{R}},$wherein r is the radius of curvature of each corner, N is the number ofcorners, and R is the radius of the maximum inscribed circle in theparticle. Each radius of curvature, r, is calculated and summed. Thisvalue is then averaged by dividing by the number of corners. Theresulting value is then divided by the radius of the maximum inscribedcircle, R. This process can be carried out manually or by usingcommercially available graphical analysis software using an SEM image at20,000 times magnification.

With reference to FIG. 9, r₁ . . . r₅ are the radii of the curvature ofeach corner and R is the radius of the maximum inscribed circle of theparticle. As an example, a perfect sphere having an average radius ofcurvature equal to the radius of the maximum inscribed circle, has aroundness index of 1.0. As the number of edges and faces in the particleincreases, the numerator of the equation decreases, and the overallroundness of the particle decreases. Roundness is discussed in detail in“Stratigraphy and Sedimentation,” 2^(nd) edition by Krumbein and Sloss(1963), which is incorporated herein by this reference for its teachingsof roundness.

In one aspect, the silica particles of the invention that have an oilabsorption value of up to 100 cc/100 g, wherein at least 80% of thesilica particles have a roundness index of at least 0.8, or morepreferably at least 0.9. Such silica particles also have a sphericity(S₈₀) factor of greater than 0.9 and a Brass Einlehner Abrasion value ofless than 8.0 mg lost/100,000 revolutions. At least 80% of theseparticles can also be classified by comparison to the silhouettes shownin FIG. 8 as rounded to well rounded, as discussed above. The processfor calculating index of roundness is as discussed above, i.e., arepresentative sample having preferably at least 20 particles in an SEMimage magnified 20,000 times is evaluated.

The silica particles of the invention that have an oil absorption valueof up to 100 cc/100 g also have a sphericity factor (S₈₀) of at least0.9. As used herein, “S₈₀” is defined and calculated as follows. An SEMimage magnified 20,000 times, which is representative of the silicaparticle sample, is imported into photo imaging software, and theoutline of each particle (two-dimensionally) is traced. Particles thatare close in proximity to one another but not attached to one anothershould be considered separate particles for the evaluation. The outlinedparticles are then filled in with color, and the image is imported intoparticle characterization software (e.g., IMAGE-PRO PLUS available fromMedia Cybernetics, Inc., Bethesda, Md.) capable of determining theperimeter and area of the particles. Sphericity of the particles canthen be calculated according to the following equation.

${{Sphericity} = \frac{{perimeter}^{2}}{4\pi \times {area}}},$wherein perimeter is the software measured perimeter derived from theoutlined trace of the particles, and wherein area is the softwaremeasured area within the traced perimeter of the particles.

The above calculation is carried out for each particle that fitsentirely within the SEM image. These values are then sorted by value,and the lowest 20% of these values are discarded. The remaining 80% ofthese values are averaged to obtain S₈₀. As an example, the sphericityfactor (Sao) for the particles shown in FIG. 5 was found to be 0.97.

Silica particles with oil absorption values greater than 100 cc/100 ggenerally were not observed to have the same high degree of sphericityand roundness as the silica particles discussed above. However, suchparticles have the ability to build viscosity and also provide superiorcleaning performance in dentifrice compositions. An exemplary image ofthese particles is shown in FIG. 7, which is sample 2F discussed inExample 2 below.

Thus, in a further aspect, the silica particles of the invention canhave an oil absorption value of greater than 100 cc/100 g. Theseparticles may not exhibit the same roundness and sphericity as thoseparticles discussed above, which have oil absorption values of up to 100cc/100 g. However, the silica particles having an oil absorption valueof greater than 100 cc/100 g are characterized as having a particle sizeof from 3 to 15 μm and will generally exhibit a Pellicle Cleaning Ratio(PCR) value at 20% silica loading of at least 85, for example from 85 to120.

The silica particles of the invention are also characterized by a numberof other properties, which are discussed below. The followingcharacteristic properties refer to both the particles having oilabsorption values up to 100 cc/100 g and greater than 100 cc/100 g,unless otherwise noted.

Median particle sizes of the silica particles of the invention weredetermined at various stages during the process and after or beforevarious particle treatment steps. As used herein, median particle size,average particle size (APS), and D₅₀, is referred to herein as theparticle size for which 50% of the sample has a smaller size and 50% ofthe sample has a larger size.

In one aspect, the silica particles of the invention have a medianparticle size while present in the liquid reaction medium of from 3 to10 μm, preferably from 3 to 8 μm, and more preferably from 4 to 6 μm. Inspecific examples, the median particle size of the silica particles inthe liquid reaction medium is from 5 to 6 μm. To determine medianparticle size of the particles in the liquid reaction medium, an aliquotof the liquid reaction medium can be removed from the recirculatingreaction zone, for example through volumetric displacement, and theparticles in the aliquot can be analyzed.

After discharging the silica product from the loop reaction zone anddrying the silica product, but prior to any milling step, the resultingsilica particles have a median particle size of from 3 to 25 μm. In someexamples, the silica particles have a median particle size after dryingbut prior to milling of from 3 to 15 μm. In further examples, the silicaparticles have a median particle size after drying but prior to millingof from 4 to 8 μm.

Milling can be used to reduce the particle size of the dried silicaparticles, as discussed above. For example, after Raymond milling or airmilling, the silica particles will generally have a median particle sizeof from 3 to 10 μm. In specific examples, the silica particle have aparticle size after milling (including Raymond milling and/or airmilling) of from 3 to 7 μm, or even from 5 to 7 μm.

Generally, it was observed that the dry particle size, sphericity, androundness of the particles was related to the structure of the silica.As the structure was lowered, a higher percentage of well rounded/highersphericity particles with little change to the liquid reaction medium(slurry) particle size distribution resulted upon drying. As thestructure was increased, the level of well rounded particles/highersphericity decreased, and the average particle size increased upondrying. Higher structure samples can be reduced to their slurry particlesizes with gentle Raymond milling. More intense Raymond milling and alsoair milling did not substantially reduce the particle size much smallerthan the slurry particle size. Milling of low structure products did notresult in much of a change in particle size. Structure of the silicaparticles generally refers to oil absorption capacity. A low structuresilica therefore has low oil absorption capacity, whereas a highstructure silica has a high oil absorption capacity.

Median particle size was determined using a Model LA-930 (or LA-300 oran equivalent) laser light scattering instrument available from HoribaInstruments, Boothwyn, Pa.

Generally, the silica particles of the invention have narrow particlesize distributions. Particle size distribution can be evaluated based ona number of parameters, including coefficient of uniformity, coefficientof curvature, and distribution symmetry. Coefficient of uniformity (Cu)is defined as D₆₀/D₁₀. Coefficient of curvature (Cc) is defined as(D₃₀/(D₁₀×D₆₀)). The peak symmetry can also be defined as(D₉₀−D₅₀)/(D₅₀−D₁₀), wherein a shape value of 1.0 would represent aperfectly symmetrical curve. Coefficients of uniformity of the silicaparticles generally range from 1.8 to 2.5. Coefficients of curvaturegenerally range from 0.2 to 0.31, while curve shape values generallyrange from 1.3 to 1.7. In specific examples, peak symmetries ranged from1.3 to 1.5, indicating a very symmetrical distribution of silicaparticles.

The silica particles of the invention have water absorption valuesranging from 57 to 272 cc water per 100 g of silica, although waterabsorption values can be made even higher. Water absorption values aredetermined with an Absorptometer “C” torque rheometer from C. W.Brabender Instruments, Inc. Approximately ⅓ of a cup of silica (orsilicate) is transferred to the mixing chamber of the Absorptometer, andit is mixed at 150 rpm. Water then is added at a rate of 6 ml/min, andthe torque required to mix the powder is recorded. As the water isabsorbed by the powder, the torque will reach a maximum as the powdertransforms from a free flowing powder to a paste. The total volume ofwater added when the maximum torque has been achieved is thenstandardized to the quantity of water that can be absorbed by 100 g ofpowder. Since the powder is used on an as received basis (it is notpreviously dried), the free moisture value of the powder is used tocalculate a “moisture corrected water AbC value” by the followingequation.

${{Water}\mspace{14mu}{Absorption}} = \frac{{{water}\mspace{14mu}{{absorbed}{\mspace{11mu}\;}({cc})}} + {\%\mspace{14mu}{moisture}}}{\left( {{100\mspace{14mu}(g)} - {\%\mspace{14mu}{moisture}}} \right)/100}$

The Absorptometer is commonly used to determine the oil number of carbonblack in compliance with ASTM D 2414 methods B and C and ASTM D 3493.

As discussed above, in one aspect, the silica particles of the inventionhave oil absorption values of up to 100 cc/100 g, for example 30 to 100cc/100 g, whereas in a further aspect, the silica particles have oilabsorption values of greater than 100 cc/100 g. for example, rangingfrom greater than 100 cc/100 g to 150 cc/100 g. Generally, the silicaparticles of the invention were observed to have oil absorptioncapacities ranging from 30 to 171 cc (cm³ or mL) of oil absorbed per 100g of silica.

Oil absorption values were measured using the rub-out method (ASTMD281). This method is based on a principle of mixing linseed oil withsilica by rubbing the linseed oil/silica mixture with a spatula on asmooth surface until a stiff putty-like paste is formed. By measuringthe quantity of oil required to have a paste mixture which will curlwhen spread out, the oil absorption value of the silica can becalculated, which represents the volume of oil required per unit weightof silica to saturate the silica sorptive capacity. A higher oilabsorption level indicates a higher structure of silica. A low value isindicative of what is considered a low-structure silica. The oilabsorption value can be determined from the following equation.

${{Oil}\mspace{14mu}{Absorption}} = {{\frac{{cc}\mspace{14mu}{oil}\mspace{14mu}{absorbed}}{{{wt}.\mspace{14mu}{silica}}\mspace{14mu}(g)} \times 100} = \frac{{ccoil}\mspace{14mu}{absorbed}}{100\mspace{14mu} g\mspace{14mu}{silica}}}$

The silica particles of the invention generally exhibit a BET surfacearea ranging from 10 to 425 m²/g. In specific examples, the silicaparticles exhibit a BET surface area ranging from 10 to 300 m²/g, andpreferably from 50 to 350 m²/g. The BET surface areas of the disclosedsilica particles was determined by the BET nitrogen adsorption method ofBrunaur et al., J. Am. Chem. Soc., 60, 309 (1938), which is incorporatedherein by reference for its teaching of the BET surface areameasurement.

The CTAB surface area of the disclosed silica particles generally rangesfrom 10 to 250 m²/g, and in some examples from 50 to 200 m²/g. CTABsurface area of silica is determined by absorption of CTAB(cetyltrimethylammonium bromide) on the silica surface, the excessseparated by centrifugation and the quantity determined by titrationwith sodium lauryl sulfate using a surfactant electrode. Specifically,about 0.5 g of silica is placed in a 250-ml beaker with 100.00 ml CTABsolution (5.5 g/L), mixed on an electric stir plate for 1 hour, thencentrifuged for 30 minutes at 10,000 rpm. One ml of 10% Triton X-100 isadded to 5 ml of the clear supernatant in a 100-ml beaker. The pH isadjusted to 3.0-3.5 with 0.1 N HCl and the specimen is titrated with0.0100 M sodium lauryl sulfate using a surfactant electrode (BrinkmannSUR1501-DL) to determine the endpoint.

The mercury (Hg) intruded volume of the disclosed silica particlesgenerally ranges from 0.5 to 3 mL/g. The mercury intruded volume ortotal pore volume (Hg) is measured by mercury porosimetry using aMicromeritics Autopore II 9220 apparatus. The pore diameters can becalculated by the Washburn equation employing a contact angle Theta (Θ)equal to 130° and a surface tension gamma equal to 485 dynes/cm. Mercuryis forced into the voids of the particles as a function of pressure andthe volume of the mercury intruded per gram of sample is calculated ateach pressure setting. Total pore volume expressed herein represents thecumulative volume of mercury intruded at pressures from vacuum to 60,000psi. Increments in volume (cm³/g) at each pressure setting are plottedagainst the pore radius or diameter corresponding to the pressuresetting increments. The peak in the intruded volume versus pore radiusor diameter curve corresponds to the mode in the pore size distributionand identifies the most common pore size in the sample. Specifically,sample size is adjusted to achieve a stem volume of 25-75% in a powderpenetrometer with a 5 mL bulb and a stem volume of about 1.1 mL. Samplesare evacuated to a pressure of 50 μm of Hg and held for 5 minutes.Mercury fills the pores from 1.5 to 60,000 psi with a 10 secondequilibrium time at each of approximately 103 data collection points.

An aqueous solution of the silica particles of the invention willgenerally exhibit a Brass Einlehner Abrasion (BEA) value of less than 10mg lost per 100,000 revolutions, preferably less than 8 mg lost per100,000 revolutions, and more preferably less than 5 mg lost per100,0000 revolutions. The BEA value will typically be at least 1.Specific ranges of BEA values include 1 to 10, 1 to 8, 1 to 7, and 1 to5 mg lost per 100,000 revolutions.

The Brass Einlehner Abrasion (BEA) test used to measure the hardness ofthe silica products of the invention is described in detail in U.S. Pat.No. 6,616,916 to Karpe et al., which is incorporated herein by referencefor its teaching of the BE Abrasion test. Generally, the test involvesan Einlehner AT-1000 Abrader used as follows: (1) a Fourdrinier brasswire screen is weighed and exposed to the action of a 10% aqueous silicasuspension for a fixed length of time; (2) the amount of abrasion isthen determined as milligrams brass lost from the Fourdrinier wirescreen per 100,000 revolutions. The result, measured in units of mgloss, can be characterized as the 10% brass Einlehner (BE) abrasionvalue.

Technidyne brightness values of the silica particles generally rangefrom 95 to 100. In specific examples, Technidyne brightness values rangefrom 97 to 100, or even 98 to 100. To measure brightness, fine powdersilica is pressed into a smooth surfaced pellet and analyzed using aTechnidyne Brightmeter S-5/BC. This instrument has a dual beam opticalsystem where the sample is illuminated at an angle of 45°, and thereflected light is viewed at 0°. It conforms to TAPPI test methods T452and T646, and ASTM Standard D985. Powdered materials are pressed toabout a 1 cm pellet with enough pressure to give a pellet surface thatis smooth and without loose particles or gloss.

Dispersions of the disclosed silica particles will generally have arefractive index (RI) value greater than 1.4. In some examples, adispersion of the disclosed silica particles have an RI value of from1.4 to 1.5. The dispersions generally have a % Transmission value (% T)ranging from 20 to 75.

To measure refractive index and degree of light transmission, a range ofglycerin/water stock solutions (about 10) was prepared so that therefractive index of these solutions lies between 1.428 and 1.460.Typically, these stock solutions will cover the range of 70 wt % to 90wt % glycerin in water. To determine the RI, one or two drops of eachstandard solution is separately placed on the fixed plate of arefractometer (Abbe 60 Refractometer Model 10450). The covering plate isfixed and locked into place. The light source and refractometer areswitched on and the refractive index of each standard solution is read.

Into separate 20-ml bottles, 2.0+/−0.01 ml of the disclosed silicaproduct was added to 18.0+/−0.01 ml of each respective stockglycerin/water solution (for products with measured oil absorption above150, the test uses 1.0 g of disclosed silica product and 19.0 g of thestock glycerin/water solution). The bottles were then shaken vigorouslyto form a silica dispersion, the stoppers were removed from the bottles,and the bottles were placed in a desiccator, which was then evacuatedwith a vacuum pump (about 24 inches Hg).

The dispersions were then de-aerated for 120 minutes and visuallyinspected for complete de-aeration. The “% T” at 590 nm (Spectronic 20D+) was measured after the samples returned to room temperature (about10 minutes), according to the manufacturer's operating instructions. The% T was measured on the disclosed silica product by placing an aliquotof each dispersion in a quartz cuvette and reading the % T at 590 nmwavelength for each sample on a 0-100 scale. The % Transmittance vs. RIof the stock solutions was plotted on a curve. The RI of the silica wasdefined as the position of the plotted peak maximum (the ordinate orX-value) on the % T vs. the RI curve. The Y-value (or abscissa) of thepeak maximum was the % T.

The silica particles can be filtered and washed with water to reduce thesodium sulfate levels (when present) to tolerable levels. Washing of thereaction product is generally conducted after filtering. The pH of thewashed wet cake can be adjusted, if necessary, prior to proceeding tosubsequent steps described herein. Sodium sulfate content of the silicaparticles of the invention can be up to about 6%. Sodium sulfate contentwas measured by conductivity of a known concentration of silica slurry.Specifically, 38 g silica wet cake sample was weighed into a one-quartmixer cup of a Hamilton Beach Mixer, model Number 30, and 140 ml ofdeionized water was added. The slurry was mixed for 5 to 7 minutes, thenthe slurry was transferred to a 250-ml graduated cylinder and thecylinder filled to the 250-ml mark with deionized water, using the waterto rinse out the mixer cup. The sample was mixed by inverting thegraduated cylinder (covered) several times. A conductivity meter, suchas a Cole Parmer CON 500 Model #19950-00, was used to determine theconductivity of the slurry. Sodium sulfate content was determined bycomparison of the sample conductivity with a standard curve generatedfrom a known method-of-addition sodium sulfate/silica compositionslurries.

Dentifrice Compositions

The silica product of the invention are particularly useful indentifrice compositions as part or all of the abrasive or cleaningagent. As used herein, a “dentifrice composition” refers to acomposition that can be used to maintain oral hygiene, for example bycleaning accessible surfaces of the teeth. The dentifrice compositioncan be a liquid, powder, or paste. Typically, the dentifricecompositions are primarily composed of water, detergent, humectant,binder, flavoring agents, and a finely powdered abrasive (the disclosedsilica product). The silica particles of the invention, whenincorporated into dentifrice compositions, can be present at a level offrom about 5% to about 50% by weight, preferably from about 10% to about50% by weight, and more preferably from about 10% to about 35% byweight. As a specific example, the dentifrice composition can comprisethe silica particles present at about 20% by weight.

Exemplary oral dentifrice or oral cleaning formulations can comprise anyone or more of the following ingredients in any suitable amount, forexample, in the following amounts (% by weight). The silica thickener inthe below example can be any thickener known in the art, such as ZEODENTproducts as discussed below, and/or can include silica particles of theinvention. The abrasive preferably contains silica particles of theinvention in the amounts shown in Table 2.

TABLE 2 Ingredients and relative amounts in an exemplary dentifricecomposition. Exemplary Dentifrice Composition Ingredient Amount (% byweight) humectant(s) (total)  5-70 Deionized water  5-70 Binder 0.5-2.0Therapeutic agent 0.1-2.0 Chelating agent 0.4-10  Thickener  0-15Surfactant 0.5-15  Abrasive 10-50 Sweetining agent <1.0 Coloring agent<1.0 Flavoring agent <5.0 Preservative <0.5

The disclosed silica particles can be utilized alone as the abrasive inthe dentifrice composition, or as an additive or co-abrasive with otherabrasive materials discussed herein or known in the art. Thus, anynumber of other conventional types of abrasive additives can be presentwithin the dentifrice compositions of the invention. Other such abrasiveparticles include, for example, precipitated calcium carbonate (PCC),ground calcium carbonate (GCC), chalk, bentonite, dicalcium phosphate orits dihydrate forms, silica gel (by itself, and of any structure),precipitated silica, amorphous precipitated silica (by itself, and ofany structure as well), perlite, titanium dioxide, dicalcium phosphate,calcium pyrophosphate, alumina, hydrated alumina, calcined alumina,aluminum silicate, insoluble sodium metaphosphate, insoluble potassiummetaphosphate, insoluble magnesium carbonate, zirconium silicate,particulate thermosetting resins and other suitable abrasive materials.Such materials can be introduced into the dentifrice compositions totailor the polishing characteristics of the target formulation.

In addition to the abrasive component, the dentifrice can also containone or more organoleptic enhancing agents. Organoleptic enhancing agentsinclude humectants, sweeteners, surfactants, flavorants, colorants andthickening agents, (also sometimes known as binders, gums, orstabilizing agents).

Humectants serve to add body or “mouth texture” to a dentifrice as wellas preventing the dentifrice from drying out. Suitable humectantsinclude polyethylene glycol (at a variety of different molecularweights), propylene glycol, glycerin (glycerol), erythritol, xylitol,sorbitol, mannitol, lactitol, and hydrogenated starch hydrolyzates, andmixtures thereof. In specific examples, humectants are present in anamount from about 20 wt % to about 50 wt % of the dentifricecomposition, for example 40%.

Sweeteners can be added to the dentifrice composition (e.g., toothpaste)to impart a pleasing taste to the product. Suitable sweeteners includesaccharin (as sodium, potassium or calcium saccharin), cyclamate (as asodium, potassium or calcium salt), acesulfame-K, thaumatin,neohesperidin dihydrochalcone, ammoniated glycyrrhizin, dextrose,levulose, sucrose, mannose, and glucose.

Surfactants can be used in the dentifrice compositions of the inventionto make the compositions more cosmetically acceptable. The surfactant ispreferably a detersive material which imparts to the compositiondetersive and foaming properties. Suitable surfactants are safe andeffective amounts of anionic, cationic, nonionic, zwitterionic,amphoteric and betaine surfactants such as sodium lauryl sulfate, sodiumdodecyl benzene sulfonate, alkali metal or ammonium salts of lauroylsarcosinate, myristoyl sarcosinate, palmitoyl sarcosinate, stearoylsarcosinate and oleoyl sarcosinate, polyoxyethylene sorbitanmonostearate, isostearate and laurate, sodium lauryl sulfoacetate,N-lauroyl sarcosine, the sodium, potassium, and ethanolamine salts ofN-lauroyl, N-myristoyl, or N-palmitoyl sarcosine, polyethylene oxidecondensates of alkyl phenols, cocoamidopropyl betaine, lauramidopropylbetaine, palmityl betaine and the like. Sodium lauryl sulfate is apreferred surfactant. The surfactant is typically present in the oralcare compositions of the present invention in an amount of about 0.1 toabout 15% by weight, preferably about 0.3% to about 5% by weight, suchas from about 0.3% to about 2.5%, by weight.

Flavoring agents can also be added to dentifrice compositions. Suitableflavoring agents include, but are not limited to, oil of wintergreen,oil of peppermint, oil of spearmint, oil of sassafras, and oil of clove,cinnamon, anethole, menthol, thymol, eugenol, eucalyptol, lemon, orangeand other such flavor compounds to add fruit notes, spice notes, etc.These flavoring agents generally comprise mixtures of aldehydes,ketones, esters, phenols, acids, and aliphatic, aromatic and otheralcohols.

Colorants can be added to improve the aesthetic appearance of theproduct. Suitable colorants include without limitation those colorantsapproved by appropriate regulatory bodies such as the FDA and thoselisted in the European Food and Pharmaceutical Directives and includepigments, such as TiO₂, and colors such as FD&C and D&C dyes.

Thickening agents are useful in the dentifrice compositions to provide agelatinous structure that stabilizes the toothpaste against phaseseparation. Suitable thickening agents include silica thickener; starch;glycerite of starch; gums such as gum karaya (sterculia gum), gumtragacanth, gum arabic, gum ghatti, gum acacia, xanthan gum, guar gumand cellulose gum; magnesium aluminum silicate (Veegum); carrageenan;sodium alginate; agar-agar; pectin; gelatin; cellulose compounds such ascellulose, carboxymethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, hydroxymethyl cellulose, hydroxymethylcarboxypropyl cellulose, methyl cellulose, ethyl cellulose, and sulfatedcellulose; natural and synthetic clays such as hectorite clays; andmixtures thereof. Typical levels of thickening agents or binders arefrom about 0 wt % to about 15 wt % of a toothpaste composition.

Useful silica thickeners for utilization within a toothpastecomposition, for example, include, as a non-limiting example, anamorphous precipitated silica such as ZEODENT 165 silica. Otherpreferred (though non-limiting) silica thickeners are ZEODENT 153, 163and/or 167 and ZEOFREE, 177, and/or 265 silica products, all availablefrom J. M. Huber Corporation.

Therapeutic agents can also be used in the compositions to provide forthe prevention and treatment of dental caries, periodontal disease andtemperature sensitivity. Examples of therapeutic agents, withoutintending to be limiting, are fluoride sources, such as sodium fluoride,sodium monofluorophosphate, potassium monofluorophosphate, stannousfluoride, potassium fluoride, sodium fluorosilicate, ammoniumfluorosilicate and the like; condensed phosphates such as tetrasodiumpyrophosphate, tetrapotassium pyrophosphate, disodium dihydrogenpyrophosphate, trisodium monohydrogen pyrophosphate; tripolyphosphates,hexametaphosphates, trimetaphosphates and pyrophosphates, such as;antimicrobial agents such as triclosan, bisguanides, such as alexidine,chlorhexidine and chlorhexidine gluconate; enzymes such as papain,bromelain, glucoamylase, amylase, dextranase, mutanase, lipases,pectinase, tannase, and proteases; quatemary ammonium compounds, such asbenzalkonium chloride (BZK), benzethonium chloride (BZT),cetylpyridinium chloride (CPC), and domiphen bromide; metal salts, suchas zinc citrate, zinc chloride, and stannous fluoride; sanguinariaextract and sanguinarine; volatile oils, such as eucalyptol, menthol,thymol, and methyl salicylate; amine fluorides; peroxides and the like.Therapeutic agents may be used in dentifrice formulations singly or incombination at a therapeutically safe and effective level.

Preservatives can also added to the compositions of the presentinvention to prevent bacterial growth. Suitable preservatives approvedfor use in oral compositions such as methylparaben, propylparaben andsodium benzoate can be added in safe and effective amounts.

The dentifrices disclosed herein can also a variety of additionalingredients such as desensitizing agents, healing agents, other cariespreventative agents, chelating/sequestering agents, vitamins, aminoacids, proteins, other anti-plaque/anti-calculus agents, opacifiers,antibiotics, anti-enzymes, enzymes, pH control agents, oxidizing agents,antioxidants, and the like.

The dentifrice composition also typically comprises a solvent, which isusually water. Generally, water provides the balance of the compositionin addition to the additives mentioned above. The water is preferablydeionized and free of impurities. The dentifrice will usually comprisefrom about 5 wt % to about 70 wt % of water, for example 5 wt % to 35 wt%, such as 11% water.

A specific example of a disclosed dentifrice composition is one thatcomprises 10-50% by weight of disclosed silica particles, Glycerin,Sorbitol, Water, CARBOWAX 600, CEKOL, Tetrasodium Pyrophosphate. SodiumSaccharin, Sodium Fluoride, ZEODENT, Titanium Dioxide, Sodium LaurylSulfate, a flavor, and an optional colorant.

The dentifrice compositions disclosed herein can be evaluated using avariety of measurements. The cleaning property of dentifricecompositions is typically expressed in terms of Pellicle Cleaning Ratio(“PCR”) value. The PCR test measures the ability of a dentifricecomposition to remove pellicle film from a tooth under fixed brushingconditions. The PCR test is described in “In Vitro Removal of Stain WithDentifrice” G. K. Stookey, et al., J. Dental Res., 61, 1236-9, 1982,which is incorporated herein by reference for its teaching of PCR.Generally, the dentifrice compositions of the invention have a PCR valueof at least 85 at loading levels of 20%, for example, from about 85 toabout 107.

The Radioactive Dentin Abrasion (RDA) of the dentifrice compositions ofthe invention will generally be at least 100, for example from about 100to about 315. RDA values of dentifrices containing the silica particlesused in this invention are determined according to the method set forthby Hefferen, Journal of Dental Res., July-August 1976, 55 (4), pp.563-573, and described in Wason U.S. Pat. Nos. 4,340,583, 4,420,312 and4,421,527, which are each incorporated herein by reference for theirteaching of RDA measurements. Both PCR and RDA results vary dependingupon the nature and concentration of the components of the dentifricecomposition. PCR and RDA values are unitless.

The toothpaste (dentifrice) viscosity of the disclosed dentifricecompositions varies and can be measured utilizing a BrookfieldViscometer Model RVT equipped with a Helipath T-F spindle and set to 5rpm by measuring the viscosity of the toothpaste at 25° C. at threedifferent levels as the spindle descends through the toothpaste testsample and averaging the results. Brookfield viscosity is expressed incentipoise (cP).

Continuous Loop Reactor

The process of the invention, in various aspects, can be carried outusing a continuous loop reactor or a pipe reactor. A suitable continuousloop reactor generally comprises an inlet port for the acidulatingagent, an inlet port for the alkali metal silicate, and a productdischarge port all in fluid communication with a continuous loop definedby one or more pipes. The liquid medium in the continuous loop can berecirculated using a variety of means, such as a pump that is in theloop itself. Other components of the continuous loop reactor can includewithout limitation a heat exchanger in the loop for controllingtemperature in the liquid medium, a back-pressure valve for controllingpressure, and/or an in-line mixing device in the loop for mixing thecontents of the liquid reaction medium.

With reference to FIG. 1, an exemplary continuous loop reactor 100comprises an acidulating agent inlet port 110 for introducing theacidulating agent into the liquid medium of the loop reaction zone andan alkali metal silicate inlet port 120 for introducing the alkali metalsilicate in the loop reaction zone. The loop reaction zone is defined byone or more pipes 130 that define a continuous loop. Various othercomponents can also be present in the continuous loop reactor 100,including a pump 140 for recirculating the liquid medium through the oneor more pipes 130. During the process of the invention, the pump 140should be in liquid communication with the liquid reaction medium. Thecontinuous loop can also be in fluid communication with an in-linemixing device 150. In the example shown in FIG. 1, the in-line mixingdevice 150 is also in fluid communication with the acidulating agentinlet port, and serves to both facilitate entry of the acidulating agentinto the continuous loop and also to mix the liquid medium inside theloop reaction zone. A heat exchanger 160 can also be present forcontrolling the temperature of the liquid medium in the continuous loop.The heat exchanger 160 is thus in thermal communication with the one ormore pipes 130 that define the continuous loop. As acidulating agent,alkali metal silicate, or another liquid as discussed above, arecontinuously added to the reaction, the liquid medium will overflow fromthe continuous loop and exit the loop reaction zone through productdischarge port 170. The product is then collected. In a specific aspect,the reaction can be fitted with one or more pressure controlling devicesin fluid communication with the one or more pipes 130, such as aback-pressure valve (not shown) for regulating the pressure inside theloop reactor.

Any suitable pump 140 can be used with the loop reactor. The in-linemixing device 150 is used in part to provide a high shear environment tothe recirculating liquid medium and is preferably is a rotor/stator typein-line mixer. Examples of useful rotor/stator mixers include SILVERSONin-line mixers, such as SILVERSON Model 450LS, manufactured by SILVERSONMachines, Inc.; or those commercially available from IKA-Works Inc.,Wilmington, N.C. 28405, and from Charles Ross and Son Company, Hauppage,N.Y. 11788, including Models ME-410/420X, and 450X.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompounds, compositions, articles, devices and/or methods claimed hereinare made and evaluated, and are intended to be purely exemplary of theinvention and are not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to ensure accuracywith respect to numbers (e.g., amounts, temperature, etc.), but someerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

Example 1 Continuous Loop Reactor

A continuous loop reactor was configured with a recycle loop wherein thereaction slurry could be circulated numerous times before it isdischarged (see FIG. 1). The recycle loop comprised of sections of fixedpipe joined together by sections of flexible hose. The internal diameterof the piping/hose was approximately 1″. On one side of the loop a pumpwas placed to circulate the reaction and on the opposite side aSILVERSON in-line mixer was installed to provide additional shear to thesystem and also to be used as an inlet port for introducing theacidulating agent. Between the pump and the mixer, a static mixer heatexchanger (KENICS Model I-Pilot-HT-EX 32 available from Chemineer, Inc.,Dayton, Ohio) was installed to provide a means to control thetemperature during production of silica. The discharge pipe, locatedafter the acidulating agent inlet port, allowed the product to dischargeas a function of the rates at which silicate and acidulating agent areadded. The discharge pipe can also be fitted with a back pressure valvethat enables the reactor system to operate at temperatures greater than100° C. The product discharge pipe can be oriented to collect productinto a tank for additional modification (e.g., pH adjustment), or it canbe discharged directly into a rotary or press type filter. Optionally,acid can also be added into the product discharge line to avoid postsynthetic pH adjustments when product is being prepared at pH's greaterthan 7.0.

Example 2 Preparation of Silica Product

Silica product was prepared using the continuous loop reactor describedin Example 1. Prior to the introduction of acidulating agent and alkalimetal silicate into the continuous loop reactor, precipitated silica,sodium sulfate, sodium silicate, and water were first added andrecirculated at 80 L/min. This is referred to herein as the liquidreaction medium, to which further acidulating agent and alkali metalsilicate can be added, as discussed above. This initial step wasperformed to fill the recycle loop with the approximate contents andconcentrations of a typical batch to thereby minimize the purging timebefore the desired silica product could be collected. It is believedthat this step also minimizes gelation of the loop reactor contents. Itshould be noted, however, that acidulating agent and alkali metalsilicate can be directly added to the loop reactor filled with onlywater without gelling or plugging the system. Thus, the liquid reactionmedium can comprise water without seed silica prior to the introductionof the acidulating agent and the alkali metal silicate.

A solution of 1.5 kg of ZEODENT 103, 1.34 kg of sodium sulfate, 11.1 Lof sodium silicate (2.65 MR, 13.3%) and 20 L of water was prepared.Approximately 15.5 L of this solution was then added to therecirculation loop of the loop reactor and it was heated to 68° C. Thecontents were recirculated at 80 L/min with a SILVERSON in-line mixer inthe recirculating loop operating at 60 Hz (3485 RPM). Sodium silicate(2.65 MR, 13.3%) and sulfuric acid (11.4%) were added simultaneously tothe loop at a silicate rate of 1.7 L/min and an acid rate sufficient tomaintain a pH of 9.5. When necessary, the acid rate was adjustedaccordingly to maintain the pH. Acid and silicate were added under theseconditions for 40 minutes to purge unwanted silica out of the systembefore the desired silica product was collected. After 40 minutes hadpassed, the collection vessel was emptied and its contents discarded.Acid and silicate were continuously added while silica product wascollected in a vessel with stirring at 40 RPM while maintaining thetemperature at approximately 60° C. (unless otherwise specified, thecollection temperature was the same as the reaction temperature). Afterthe desired quantity of silica product was collected, addition of acidand silicate was stopped. The contents of the loop were allowed tocirculate. The silica product in the collection vessel was adjusted topH 5.0 with the manual addition of sulfuric acid and was then filteredand washed to a conductivity of approximately 1500 μS and subsequentlydried.

Samples 2B to 2E were performed under the conditions shown in Table 3.

Samples 2F to 2S were performed according to Sample 2A with theexception that no pH adjustment was made prior to the washing/filtrationstep. Prior to drying, the pH of the product was adjusted to 5.5 withthe manual addition of dilute sulfuric acid.

Sample 2J was performed according to Sample 2F with the exception thatthe pH was adjusted to 6.5 prior to drying.

Sample 2N was performed with the continuous loop reactor as describedabove with the exception that the stator was removed from the SILVERSONin line mixer.

TABLE 3 Summary of reaction conditions for Sample 2A to 2S. SilicateSulfuric Sodium Recirc. Mean Addition Rxn Acid Silicate Rate NumberSilverson Rate Temp Sample (%) (%) (L/min) of Passes RPM (L/min) pH (°C.) 2A 11.4 13.3 80 29 3485 1.7 9.5 68 2B 11.4 13.3 43 16 3485 1.7 9.570 2K 11.4 13.3 80 21 1743 2.6 9.5 58 2C 11.4 13.3 80 21 3600 2.6 9.5 582D 11.4 13.3 80 20 1743 2.6 9.5 93 2E 11.4 13.3 80 20 3485 2.6 9.5 93 2F11.4 13.3 80 19 3485 2.6 7.2 43 2O 5.7 6.7 80 26 3485 1.7 7.5 68 2G 17.019.5 80 28 3485 1.7 7.5 68 2L 11.4 13.3 72 18 1743 2.6 7.2 33 2M 11.413.3 80 29 1743 1.7 7.3 94 2N 11.4 13.3 80 28 1743 1.7 7.5 94 2J 17.019.5 77 30 3485 1.7 7.5 122 2H 11.4 13.3 80 28 1743 1.7 5.5 45 2I 11.413.3 80 27 1743 1.7 2.5 44 2P 17.0 19.5 20 19 3485 0.56 7.6 95 2Q 17.019.5 40 37 3485 0.56 7.5 95 2R 17.0 19.5 60 71 3485 0.56 8.2 95 2S 11.413.3 80 29 3485 1.7 7.0 94

With reference to Table 3, the acidulating agent and the alkali metalsilicate were added at a given rate and maintained at a given percentagerelative to the liquid reaction medium. The acidulating agent wassulfuric acid, and the alkali metal silicate was sodium silicate.

The mean number of passes, or the approximate number of times a givenparticle will travel around the precipitation loop before it isdischarged, can be calculated in the following manner. With reference tothe equations shown below, the residence time of the silica product inthe recirculation loop before discharge is calculated by dividing thesystem volume by the raw material rate (silicate addition rate+acidaddition rate). The number of passes/minute can then be calculated bydividing the recirculation rate by the system volume. The residence timecan then be multiplied by the number of passes/minute to get the meannumber of passes.

${{residence}\mspace{14mu}{{time}\left( \min \right)}} = \frac{{system}\mspace{14mu}{{volume}(L)}}{{combined}\mspace{14mu}{raw}\mspace{14mu}{material}\mspace{14mu}{addition}\mspace{14mu}{{rate}\left( {L\text{/}\min} \right)}}$${{number}\mspace{14mu}{of}\mspace{14mu}{passes}\text{/}\min} = \frac{{recirculation}\mspace{14mu}{{rate}\left( {L\text{/}\min} \right)}}{{system}\mspace{14mu}{{volume}(L)}}$${{residence}\mspace{14mu}{{time}\left( \min \right)} \times \frac{{number}\mspace{14mu}{of}\mspace{14mu}{passes}}{\left( \min \right)}} = {{mean}\mspace{14mu}{number}{\mspace{11mu}\;}{of}\mspace{14mu}{passes}}$

As the mean number of passes was increased, the sphericity and roundnesscharacteristics of the particles improved.

Generally, the continuous loop reactor was easily able to maintain agiven condition during the reaction. As discussed above, at a givensilicate flow rate, the rate of acid was adjusted to achieve a desiredpH. Once the acid rate had stabilized, continuous operation at a desiredcondition could be maintained. Adjustment of pH was achieved bymodifying the rate of acid addition. Conditions ranging from pH 2.5 to9.5 and temperatures ranging from 24 to 122° C. have been specificallytested and no blocking or gelling of the liquid reaction medium wasobserved.

Example 3 Silica Particles Prepared from Example 2

The silica products prepared in Example 2 were characterized. Thereaction slurry particle size (particle size of particles in the recycleloop) under most reaction conditions tested was generally found to beabout 4-8 μm, with most examples falling in the 4-6 μm range. The dryparticle size and sphericity/roundness of the particles was directlyrelated to the structure of the silica. As the structure was lowered, ahigher percentage of unagglomerated particles with high sphericity androundness with little change from the slurry particle size distributionresulted upon drying. As the structure was increased, the level ofparticle agglomeration increased, the sphericity and roundness of theparticles decreased, and the average particle size increased upondrying.

Higher structure samples can be reduced to their slurry particle sizeswith gentle Raymond milling. More intense Raymond milling and also airmilling did not substantially reduce the particle size much smaller thanthe slurry particle size. Milling of low structure products did notresult in much of a change in particle size. The particle sizedistributions of the silica produced by the continuous process wereGaussian and typically less broad than precipitated silica prepared byconventional processes. Slurry, spray dried, Raymond milled and airmilled particle sizes of the particles prepared with the continuous loopreactor are shown in Table 4. For the remaining examples, unmilled driedsilica samples are designated with a “−1,” Raymond milled samples with a“−2” and air milled samples with a “−3.” Particle size distributions forsilica product prepared by the continuous loop process and conventionalprocesses are shown in FIG. 2.

TABLE 4 Slurry, spray dried and milled particle size of silica productsprepared by continuous loop process. Slurry Raymond Air milled APS DryAPS milled APS APS (Horiba (Horiba (Horiba (Horiba median, median,median, median, Sample μm) μm) μm) μm) 2A 4.8 8.8 — — 2B 5.4 10.3 6.25.4 2C 5.3 12.2 5.4 — 2D 4.5 4.8 — 5.0 2E 3.8 5.3 — 4.4 2F 6.4 18.1 5.75.7 2G 5.1 9.3 5.9 5.4 2H — 26.1 11.7  — 2I — 19.6 10.5  — 2J 4.2 5.5 —4.6 2K 5.4 12.4 — — 2L 8.2 20.4 — — 2M 5.7 8.0 — — 2N 4.7 8.0 — — 2O 4.714.2 — — 2P — 7.2 — — 2Q 5.0 6.3 — — 2R 4.3 5.4 — — 2S 4.6 7.5 — —

The reaction conditions described above and listed in Table 3 allowedfor the production of exemplary silica products with “low” to “mediumhigh structures,” with oil absorption values generally ranging from 32to 171 cc/100 g. Moisture corrected water AbC values for the silicaproducts produced ranged from 57 to 272 cc/100 g. CTAB surface areasranged from 10 to 250 m²/g. The BET surface areas, which ranged from 17to 425, were higher than typical precipitated silica materials producedby the conventional batch processes. The brightness values for thesilica products prepared by the continuous process were very good, whichis likely attributable to their high sphericity and roundness. Thesilica products produced by the continuous process disclosed hereinexhibited brightness values typically greater than 96, with theexception of those prepared at pH's less than 7. The physical propertiesof the silica products prepared by the disclosed process are shown inTable 5.

TABLE 5 Physical properties of continuous reactor samples. Moisture BETCTAB Hg Corrected Oil Surface Surface Intruded Water AbC Absorption AreaArea Na₂SO₄ H₂O Volume 5% Technidyne Sample (cc/100 g) (cc/100 g) (m²/g)(m²/g) (%) (%) (ml/g) pH Brightness 2A-1 79 68 232 50 5.23 6.5 1.79 4.697.7 2B-1 114 88 207 80 0.74 7.1 1.81 8.5 98.6 2B-2 100 64 120 52 0.517.5 1.30 8.7 97.5 2B-3 101 74 120 66 0.51 8.0 0.82 8.7 97.8 2C-1 139 106353 95 0.35 7.9 2.06 8.9 98.3 2C-2 109 83 178 98 0.35 7.6 0.74 9.0 96.42D-1 80 60 133 29 0.35 6.5 1.13 8.7 98.2 2D-3 75 60 55 28 0.35 8.1 1.098.9 98.3 2E-1 78 60 219 32 0.35 7.7 1.16 7.3 98.3 2E-3 70 58 149 28 0.358.1 0.99 7.7 98.1 2F-1 212 134 383 194 3.97 6.3 2.85 7.1 97.5 2F-2 150125 376 185 3.66 6.6 2.37 7.4 96.8 2F-3 157 130 247 187 3.1 6.9 2.25 7.297.1 2G-1 87 54 157 48 2.71 5.0 1.36 7.3 98.8 2G-2 81 53 121 78 2.16 6.01.08 7.6 96.6 2G-3 79 67 162 68 2.32 5.8 1.10 7.5 98.1 2H-1 272 171 361250 1.1 8.6 3.24 8.5 94.2 2H-2 203 158 310 246 0.7 8.3 2.65 8.5 93.22I-1 215 160 374 232 0.4 9.0 3.11 8.5 97.2 2I-2 192 140 413 219 0.4 8.93.31 8.5 96.8 2J-1 57 32 17 10 0.9 4.2 0.63 8.4 95.4 2K-1 140 101 279 980.35 8.7 2.15 8.7 98.7 L2-1 204 148 425 217 2.9 7.4 2.72 7.3 96.8 L2-2158 125 138 210 2.6 7.2 1.30 7.4 96.7 2M-1 76 62 70 50 1.6 4.6 1.03 7.498.0 2M-2 79 59 77 54 1.6 7.2 1.10 7.4 96.8 2N-1 75 59 59 47 1.6 4.61.08 7.4 96.7 2N-2 66 51 61 49 1.6 4.0 0.75 7.3 97.0 2O-1 138 101 166 832.39 5.8 2.35 6.4 98.5 2P-1 67 56 49 29 2.0 6.5 0.88 7.4 97.8 2Q-1 61 5124 16 1.5 5.5 0.71 8.0 97.6 2R-1 59 54 39 21 1.7 4.8 0.66 7.8 97.9 2S-182 61 95 38 1.92 5.0 1.22 7.6 97.8

Particle size distributions of the exemplary silica particle batchesprepared using the continuous process disclosed herein were alsoevaluated. The results are shown in Table 6. Coefficient of uniformity(Cu) is defined as D₆₀/D₁₀. Coefficient of curvature (Cc) is defined as(D₃₀/(D₁₀×D₆₀)). The peak symmetry is defined as (D₉₀−D₅₀)/(D₅₀−D₁₀),wherein a peak symmetry value of 1.0 would represent a perfectlysymmetrical distribution.

TABLE 6 Particle size distribution properties. Coefficient ofCoefficient of Peak Sample Uniformity Curvature Symmetry 2B-2 2.47 0.231.48 2B-3 2.37 0.26 1.60 2C-2 2.33 0.26 1.60 2C-3 2.43 0.29 1.35 2E-22.22 0.30 1.43 2F-2 1.98 0.23 1.44 2F-3 2.20 0.24 1.44

Scanning electron micrographs of the silica products prepared by thecontinuous process disclosed herein exhibited a much more spherical andhomogeneous distribution relative to conventional silica. The level ofsphericity/roundness was generally greater with low structure products,since they did not agglomerate as readily upon drying. As the structurelevel increased, the degree of sphericity/roundness and homogeneity ofthe particles decreased. When comparing silica products prepared withthe continuous loop process to those produced by traditional batchtechnology, the differences in sphericity and roundness can be clearlyobserved. Scanning electron micrographs of low, medium, and medium-highstructure silica products produced by the continuous loop reactor andthose prepared by traditional batch processes are shown in FIGS. 3-6.

Modification to the level of shear imparted into the system by theSILVERSON in line mixer was also studied. Adjustment of the power inputfrom 30 to 60 hz and removal of the stator from the SILVERSON in linemixer did not substantially impact the quality of the sphericity androundness of the particles produced. The mean number of passes, however,correlated with sphericity and roundness of the particles. Samples 2P,2Q and 2R were performed under similar conditions with the exceptionthat the recirculation rate (and mean number of passes) was varied.Sample 2R, with the highest mean number of passes (71), was found tohave the highest quality sphericity and particle roundness compared toSamples 2P and 2Q.

Example 4 Silica Particles Prepared from Different Acidulating Agents

-   -   (i) 4A.

A solution comprised of 1.5 kg of ZEODENT 103, 1.34 kg of sodiumsulfate, 11.1 L of sodium silicate (2.65 MR, 13.3%) and 20 L of waterwas prepared. Approximately 15.5 L of this solution was then added tothe recirculation loop of the loop reactor described in Example 1 and itwas heated to 50° C. The contents were recirculated at 78 L/min with aSILVERSON in-line mixer in the recirculation loop operating at 60 Hz(3485 RPM). Sodium silicate (2.65 MR, 13.3%) and carbon dioxide (99.9%)were added simultaneously to the loop at a silicate rate of 0.5 L/minand a carbon dioxide rate sufficient to maintain a pH of 9.3 (theapproximate flow rate was 47 L/min). When necessary, the carbon dioxideflow rate was adjusted accordingly to maintain the pH. Carbon dioxideand silicate were added under these conditions for 40 minutes to purgeunwanted silica out of the system before the desired material wascollected. After 40 minutes had passed, the collection vessel wasemptied and its contents discarded. Carbon dioxide and silicate werecontinuously added while silica product was collected in a vessel withstirring at 40 RPM while maintaining the temperature at approximately50° C. After the desired quantity of product was collected, addition ofcarbon dioxide and silicate was stopped. The contents of the loop wereallowed to circulate. The silica product in the collection vessel wasadjusted to pH 6.0 with the manual addition of sulfuric acid and wasthen filtered, washed to a conductivity of approximately 1500 μS, wasdried and milled, if necessary.

(ii) 4B.

Example 4B was performed according to the method of Example 4A with theexception that the sodium silicate contained 10% by weight sodiumsulfate, the pH was maintained at 8.5 with an approximate carbon dioxideflow rate of 64 L/min.

(ii) 4C.

A solution comprised of 1.5 kg of ZEODENT 103, 1.34 kg of sodiumsulfate, 11.1 L of sodium silicate (2.65 MR, 13.3%) and 20 L of waterwas prepared. Approximately 15.5 L of this solution was then added tothe recirculation loop of the loop reactor and it was heated to 43° C.The contents were recirculated at 80 L/min with a SILVERSON in-linemixer in the recirculation loop operating at 60 Hz (3485 RPM). Sodiumsilicate (2.65 MR, 13.3%) and sulfuric acid (11.4%) containing sodiumsulfate at a concentration of 23 g/L were added simultaneously to theloop at a silicate rate of 2.55 L/min and an acid rate sufficient tomaintain a pH of 7.5. When necessary, the acid rate was adjustedaccordingly to maintain the pH. Acid (containing sodium sulfate) andsilicate were added under these conditions for 40 minutes to purgeunwanted silica out of the system before the desired material wascollected. After 40 minutes had passed, the collection vessel wasemptied and its contents discarded. Acid (containing sodium sulfate) andsilicate were continuously added while silica product was collected in avessel with stirring at 40 RPM while maintaining the temperature atapproximately 45° C. After the desired quantity of product wascollected, addition of acid and silicate was stopped. The contents ofthe loop were allowed to circulate. The silica product in the collectionvessel was then filtered and washed to a conductivity of approximately1500 μS. Prior to spray drying, the pH was adjusted to pH 6.0 with themanual addition of sulfuric acid.

(iv) 4D.

Example 4D was performed according to Example 4C with the exception thatthe silicate rate was 1.7 L/min, the pH was maintained at 7.1, thereaction temperature was 95° C. and the collection temperature wasmaintained at approximately 90° C.

(v) 4E.

Example 4E was performed according to Example 5D with the exception thatthe silicate concentration was 19.5%, 17% sulfuric acid containingaluminum sulfate at a concentration of 8.5 g/L, the reaction temperaturewas 40° C. and the pH was maintained at 7.5.

TABLE 7 Physical properties of silica samples prepared in Example 4. BETCTAB Hg Median Water Oil Surface Surface Intruded Particle AbCAbsorption Area Area Na₂SO₄ H₂O Volume 5% Size Technidyne Ex (cc/100 g)(cc/100 g) (m²/g) (m²/g) (%) (%) (ml/g) pH (μm) Brightness 4A-1 76 134193 194 5.6 8.4 1.14 7.7 6.5 99.1 4B-1 77 128 16 19 — — — — 6.4 — 4C-2155 121 424 186 4.0 5.4 2.02 7.1 5.8 97.1 4D-1 81 60 94 45 2.0 4.9 0.787.6 7.6 97.4 4E-2 119 104 358 164 3.6 6.5 1.28 7.5 5.7 97.8

In addition to sulfuric acid, additives and other acidulating agents canbe used in the continuous loop reactor to produce precipitated silica.Examples 4A and 4B used carbon dioxide in the place of sulfuric acid asthe acidulating agent. This was accomplished by introducing the gas intothe continuous loop reactor through the SILVERSON mixer. Slower silicaterates (0.5 L/min) were used in these examples in order to give thecarbon dioxide that was introduced sufficient time to react and maintainthe desired pH, since the flow of carbon dioxide was limited. Since thecarbon dioxide produces a weak acid (carbonic acid), pH targets ofgreater than 8.5 were used. The silica products resulting from Example4A had high sphericity and roundness, as observed by (SEM). Raymond orair milling was not needed to achieve median particles sizes in the 5 to7 μm range. Example 4C, 4D and 4E utilized a mixture of aqueous sodiumsulfate and sulfuric acid as the acidulating agent and the physicalproperties are shown in Table 7.

Example 5 Dentifrice Compositions

Dentifrice compositions comprising the disclosed silica particles wereprepared. A number of important properties for silica products useful indentifrice compositions were evaluated. Einlehner abrasion values forthe exemplary silica particles produced by the disclosed continuousprocess were significantly lower than expected, ranging from 1.8 to 8.1mg lost/100 k rev. With conventional precipitated silica products, asthe structure decreases, the Einlehner values typically increase. Withthe disclosed continuous process silica products, this trend was notobserved. Einlehner values were consistent with particle size. Perspexabrasion values of the exemplary silica products tested were also muchlower than expected, ranging from 3.3 to 8.7.

Percent transmission (% T) values ranged from about 20 to 80% by the 4%in sorbitol test method. Refractive index (RI) values greater than 1.439were observed for all samples prepared. The increase in RI values overtypical precipitated silica products was likely due to lower reactiontemperatures. Powder RDA values for four samples tested ranged from 105to 221 as tested using the Hefferren method. This test was performed bythe Indiana University School of Dentistry.

The continuous process was also found to be useful for preparing silicaproducts that are compatible with cationic ingredients, such ascetylpyridinium chloride (CPC). CPC is a cationic anti-microbial agentthat is used in mouthwash formulations to reduce plaque, tartar andgingivitis. Conventional silica materials are not typically compatiblewith CPC due to the strong interaction between the cationic moiety ofthe CPC molecule and the negatively charged silica surface. In order toimprove the compatibility of silica with CPC, very low structure silicaproducts can be prepared with reduced available surface area for CPCbinding. The production of CPC compatible silica products byconventional batch techniques can be problematic, since increased batchtimes are typically needed to achieve the necessary structure, and lowbrightness values can result from the milling of such highly densesilica. The use of the disclosed continuous process allows for lowstructure silica products to be prepared at acceptable production ratesand with very good brightness values since hammer or air milling is notneeded to achieve the desired particle size range. A summary of thedental silica testing performed is shown in Table 8.

TABLE 8 Abrasion and optical data from silica products prepared by thecontinuous process. Einlehner Perspex (mg Abrasion RI % T lost/100k(gloss Powder (at max (4% in Sample rev) reduction) RDA % T) sorbitol)2A-1 2.5 3.8 221 1.448 58.8 2B-1 2.9 4.5 — 1.444 56.4 2B-2 3.2 — 1691.439 48.2 2B-3 4.5 — — 1.439 46.0 2K-1 3.5 7.0 — 1.439 64.0 2C-1 5.25.2 — 1.439 64.4 2D-1 3.3 7.8 — 1.439 22.8 2D-3 5.4 — — 1.435 24.4 2E-11.8 3.9 130 1.439 20.0 2E-3 3.0 — — 1.435 26.0 2S-1 4.7 8.7 — 1.439 42.82F-1 6.0 — — 1.453 70.3 2F-2 4.3 — 105 1.447 60.2 2F-3 4.1 — — 1.44755.8 4C-1 2.1 — — 1.453 80.6 4C-2 3.6 — — 1.453 72.3 4C-3 3.6 — — 1.45373.0 4C-1 4.2 — — 1.444 46.2 2O-1 3.9 — — 1.444 70.4 2G-1 1.8 — — 1.43956.1 2G-2 2.2 — — 1.439 54.2 2G-3 3.8 — — 1.439 54.5 4A-1 3.6 — — 1.43945.8 2L-1 2.3 — — — — 2L-2 2.8 — — — — 2M-1 5.8 — — — — 2M-2 6.2 — — — —2N-1 7.5 — — — — 2N-2 7.0 — — — — 2J-1 7.9 — — — — 2P-1 7.1 — — 2Q-1 8.1— — 2R-1 5.6 — —

Several samples with structures that spanned the structure range wereselected for formulation into toothpaste for PCR, RDA and REA testing.Samples were formulated into dentifrice at 20% loading and also at lowerloading levels in combination with traditional silica materials. Theformulations are shown in Tables 9-12. Several of these samples and arange of others were placed into two different formulations fordentifrice stability evaluation.

TABLE 9 Toothpaste formulation. Example Batch Formulation Number 5 A B CD E F G H I Glycerin, 99.5% 11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0Sorbitol, 70.0% 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 DeionizedWater 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 CARBOWAX 600 3.0 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 CEKOL 500T 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.21.2 Tetrasodium 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Pyrophosphate SodiumSaccharin 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Sodium Fluoride 0.2430.243 0.243 0.243 0.243 0.243 0.243 0.243 0.243 Zeodent 165 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 Zeodent 103 20.0 — 10.0 — — — — — — Zeodent 113— 20.0 10.0 — — — 10.0 10.0 10.0 2F-2 — — — 20.0 — — 10.0 — — 2B-2 — — —— 20.0 — — 10.0 — 2E-1 — — — — — 20.0 — — 10.0 Titanium Dioxide 0.5 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 Sodium Lauryl 1.2 1.2 1.2 1.2 1.2 1.2 1.21.2 1.2 Sulfate Flavor 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65Total 100 100 100 100 100 100 100 100 100

TABLE 10 Toothpaste formulation. Example Batch Formulation Number 5 J KL M N O P Q Glycerin, 99.5% 11.2 11.2 11.2 11.2 11.2 11.2 11.2 11.2Sorbitol, 70.0% 36.4 36.4 36.4 36.4 36.4 36.4 36.4 36.4 Deionized Water18.8 18.8 18.8 18.8 18.8 18.8 18.8 18.8 CARBOWAX 600 3.0 3.0 3.0 3.0 3.03.0 3.0 3.0 CEKOL 2000 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Tetrasodium 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 Pyrophosphate Sodium Saccharin 0.2 0.2 0.20.2 0.2 0.2 0.2 0.2 Sodium Fluoride 0.243 0.243 0.243 0.243 0.243 0.2430.243 0.243 Zeodent 165 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 Zeodent 103 20.0— — — — — — — Zeodent 124 — 20.0 — — — — — — Zeodent 113 — — 20.0 — — —— — 2B-2 — — — 20.0 — — — — 2E-1 — — — — 20.0 — — — 2E-3 — — — — — 20.0— — 2G-1 — — — — — — 20.0 — 2G-3 — — — — — — — 20.0 Titanium Dioxide 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 Sodium Lauryl Sulfate 1.2 1.2 1.2 1.2 1.21.2 1.2 1.2 Flavor 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 Total 100 100100 100 100 100 100 100

TABLE 11 Toothpaste formulation. Example Batch Formulation Number 5 R ST U V W X Glycerin, 99.5% 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Sorbitol, 70.0%57.36 57.36 57.36 57.36 57.36 57.36 57.36 Deionized Water 11.0 11.0 11.011.0 11.0 11.0 11.0 Carbowax 600 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Cekol 20000.8 0.8 0.8 0.8 0.8 0.8 0.8 Sodium Saccharin 0.2 0.2 0.2 0.2 0.2 0.2 0.2Sodium Fluoride 0.24 0.24 0.24 0.24 0.24 0.24 0.24 Zeodent 113 20.0 — —— — — — 2B-1 — 20.0 — — — — — 2B-2 — — 20.0 — — — — 2C-1 — — — 20.0 — —— 2C-2 — — — — 20.0 — — 2F-2 — — — — — 20.0 — 4C-2 — — — — — — 20.0Color, Blue 1.0% solution 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Sodium LaurylSulfate 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Flavor 1.0 1.0 1.0 1.0 1.0 1.0 1.0Total 100 100 100 100 100 100 100

TABLE 12 Toothpaste Formulation. Example Batch Formulation Number 5 Y ZAA AB AC AD AE Glycerin, 99.5% 11.0 11.0 11.0 11.0 11.0 11.0 11.0Sorbitol, 70.0% 40.0 40.0 40.0 40.0 40.0 40.0 40.0 Deionized Water 20.020.0 20.0 20.0 20.0 20.0 20.0 Carbowax 600 3.0 3.0 3.0 3.0 3.0 3.0 3.0Cekol 500T 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Tetrasodium 0.5 0.5 0.5 0.5 0.50.5 0.5 Pyrophosphate Sodium Saccharin 0.2 0.2 0.2 0.2 0.2 0.2 0.2Sodium Fluoride 0.243 0.243 0.243 0.243 0.243 0.243 0.243 Zeodent 1651.5 1.5 1.5 1.5 1.5 1.5 1.5 Zeodent 103 20.0 — — — — — — Zeodent 113 — —— 10.0 15.0 — — 2G-2 — 20.0 — — — — — 2H-2 — — — — — 20.0 15.0 2J-1 — —20.0 10.0 5.0 — — Titanium Dioxide 0.5 0.5 0.5 0.5 0.5 0.5 0.5 SodiumLauryl Sulfate 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Flavor 0.65 0.65 0.65 0.650.65 0.65 0.65 Total 100 100 100 100 100 100 100

Properties of the toothpaste formulations listed Tables 9-12 are shownin Table 13. The toothpaste samples prepared were found to haveacceptable aesthetic properties after 6 weeks of ageing at 25° C. Thefluoride availability values were all greater than 85% after the sameperiod of time. Viscosity build of the silica products produced by thecontinuous silica process was similar to a low structure silica for allsamples with the exception of Examples 5W and 5X, which were moreefficient at building viscosity than ZEODENT 113.

PCR, RDA and REA values were measured for a range of silica products.The PCR values ranged from 83 (Example 5AE) to 107 (Example 5AA) for thesamples tested. When formulated at loading levels of 10 to 15%, PCRvalues were generally in the 90-100 range. Dentifrice RDA values rangedfrom 94 to 315, depending upon the structure and the loading level ofthe silica tested. Example 5AA, a 20% loading of the silica prepared in2J, had the highest RDA value of 315. This was the lowest structuresilica product produced and thus was the most abrasive. When formulatedin combination with traditional silica materials, such as ZEODENT 113,at loading levels in the 5 to 10% loading range, cleaning improvementsover ZEODENT 113 alone were observed. Several silica products producedby the continuous loop reactor at higher structure levels were alsotested and found to have PCR values similar to a traditional highcleaning silica materials (Examples 5X and 5W) and were found to buildviscosity more efficiently than dentifrice containing 20% loading ofZEODENT 113 (Example 5R). The cleaning properties of higher structuresilica products prepared with the continuous loop reactor exhibited muchhigher PCR and RDA values than traditional medium to high structuresilica materials. The silica products in Examples 5X and 5W exhibit abifunctional nature, in that they provide very good cleaning whileproviding a sufficient viscosity build.

REA values for the low to medium structure silica products produced bythe continuous loop reactor were lower or equal to those of ZEODENT 113,indicating the spherical nature of these materials may be less abrasiveto enamel than traditional high cleaning silica materials, such asZEODENT 103. Dentifrice testing data is summarized in Table 13.

TABLE 13 Dentifrice data for formulations shown in Tables 7-10. 6 WeekFluoride 1 Week 3 Week 6 Week Availability 25% Viscosity ViscosityViscosity At 25° C. Example pH (cps) (cps) (cps) (%) PCR RDA REA 5A — —— — — 98 156 7.2 5B — — — — — — — 5.8 5C — — — — — 94 129 — 5D — — — — —96 122 4.6 5E — — — — — 92 133 4.7 5F — — — — — 99 174 5.8 5G — — — — —88 113 — 5H — — — — — 92 125 — 5I — — — — — 100 161 — 5J 7.5 319,000370,000 354,000 93 — — — 5K 7.4 505,000 601,000 631,000 92 — — — 5L 7.2997,000 895,000 1,036,000   91 — — — 5M 8.0 493,000 531,000 591,000 90 —— — 5N 7.3 241,000 262,000 300,000 88 — — — 5O 7.3 275,000 293,000319,000 86 — — — 5P 7.6 317,000 379,000 380,000 88 — — — 5Q 7.4 299,000319,000 362,000 89 — — — 5R 6.6 222,000 257,000 260,000 98 — — — 5S 8.2144,000 156,000 170,000 100  — — — 5T 8.1 143,000 155,000 171,000 100  —— — 5U 8.0 195,000 206,000 242,000 97 — — — 5V 8.0 175,000 187,000196,000 95 — — — 5W 7.6 288,000 317,000 328,000 93 — — — 5X 7.4 353,000384,000 370,000 92 — — — 5Y — — — — — 100 228 — 5Z — — — — — 104 261 —5AA — — — — — 107 315 — 5AB — — — — — 104 290 — 5AC — — — — — 103 231 —5AD — — — — — 87 111 — 5AE — — — — — 83  94 —

Example 6 Preparation of Sodium Alumino Silicate and Sodium MagnesiumAlumino Silicate

(i) 6A

A solution of 1.5 kg of ZEODENT 103, 1.34 kg of sodium sulfate, 11.1 Lof sodium silicate (3.32 MR, 20.0%) and 20 L of water was prepared.Approximately 15.5 L of this solution was then added to therecirculation loop of the loop reactor described in Example 1 and it washeated to 60° C. The contents were recirculated at 80 L/min with aSILVERSON in-line mixer in the recirculating loop operating at 60 Hz(3485 RPM). Sodium silicate (3.32 MR, 20.0%) and aqueous aluminumsulfate (11.4%) were added simultaneously to the loop at a silicate rateof 1.7 L/min and an aluminum sulfate rate sufficient to maintain a pH of8.5. When necessary, the acid rate was adjusted accordingly to maintainthe pH. Acid and silicate were added under these conditions for 40minutes to purge unwanted silica out of the system before the desiredmaterial was collected. After 40 minutes had passed, the collectionvessel was emptied and its contents discarded. Acid and aluminum sulfatewere continuously added while silicate product was collected in a vesselwith stirring at 40 RPM while maintaining the temperature atapproximately 60° C. After the desired quantity of product wascollected, addition of aluminum sulfate and silicate was stopped. Thecontents of the loop were allowed to circulate. The silicate product inthe collection vessel was then filtered, washed to a conductivity ofapproximately 1500 μS and was dried.

(ii) 6B.

Example 6B was performed according to Example 6A with the exception thatrecirculation rate was 77 L/min and the reaction temperature was 36° C.and the collection vessel temperature was held at ambient temperature.The sample was Raymond milled after drying.

(iii) 6C.

Example 6C was performed according to Example 6B with the exception thatthe static mixer heat exchanger was removed from the apparatus and thereaction temperature was 32° C.

(iv) 6D.

Example 6D was performed according to Example 6C with the exception thatthe aqueous aluminum sulfate concentration was 14.5%, a silicate rate of3.4 L/min and a reaction temperature of 24° C.

-   -   (v) 6E.

The static mixer heat exchanger was removed from the loop reactor. Asolution comprised of 1.5 kg of ZEODENT 103, 1.34 kg of sodium sulfate,11.1 L of sodium silicate (3.32 MR, 20.0%) and 20 L of water wasprepared. Approximately 15.5 L of this solution was then added to therecirculation loop of the loop reactor and it was heated to 39° C. Thecontents were recirculated at 110 L/min with a SILVERSON in-line mixerin the recirculation loop operating at 60 Hz (3485 RPM). Sodium silicate(3.32 MR, 20.0%) containing magnesium hydroxide at 4.5 g/L and aqueousaluminum sulfate (34.0%) were added simultaneously to the loop at asilicate rate of 2.5 L/min and an aqueous aluminum sulfate ratesufficient to maintain a pH of 8.8. When necessary, the aqueous aluminumsulfate rate was adjusted accordingly to maintain the pH. Aqueousaluminum sulfate and silicate containing magnesium hydroxide were addedunder these conditions for 25 minutes to purge unwanted silica out ofthe system before the desired material was collected. After 25 minuteshad passed, the collection vessel was emptied and its contentsdiscarded. Aqueous aluminum sulfate and silicate containing magnesiumhydroxide were continuously added while silicate product was collectedin a vessel with stirring at 40 RPM while maintaining the temperature atapproximately 39° C. After the desired quantity of product wascollected, addition of aqueous aluminum sulfate and silicate containingmagnesium hydroxide was stopped. The contents of the loop were allowedto circulate. The silicate product in the collection vessel was thenfiltered, washed to a conductivity of approximately 1500 μS and wasdried.

TABLE 14 Physical properties for silica products prepared in Example 6.BET CTAB Hg Median Water Oil Surface Surface Intruded Particle AbCAbsorption Area Area Na₂SO₄ H₂O Volume 5% Size Technidyne Ex (cc/100 g)(cc/100 g) (m²/g) (m²/g) (%) (%) (ml/g) pH (μm) Brightness 6A-1 79 68232 50 5.2 6.5 1.30 4.7 8.8 97.7 6B-1 91 68 198 109 0.1 6.5 0.53 8.411.7 99.2 6B-2 79 68 180 80 0.1 7.3 0.80 8.4 5.7 98.0 6C-1 91 78 222 930.1 7.7 1.16 7.9 9.9 99.3 6C-2 83 60 178 86 0.1 7.4 1.67 8.0 6.6 98.96D-2 137 122 272 160 1.1 8.4 1.02 9.6 6.4 97.6 6E-1 115 68 369 153 0.310.8 1.53 10.3 10.7 98.4 6E-2 119 68 213 174 0.3 10.2 0.96 10.2 6.2 97.8

Examples 6A, 6B, 6C and 6D describe the preparation of sodium aluminosilicates in the continuous loop reactor by the neutralization of sodiumsilicate with aqueous aluminum sulfate. The aqueous aluminum sulfate wasintroduced into the loop reactor through the SILVERSON in line mixer.Modification of the number of passes was used to produce a range ofproducts with oil absorption values ranging from approximately 60 to 122cc/100 g. Example 6E describes the preparation of sodium magnesiumalumino silicate by the neutralization of sodium silicate/magnesiumhydroxide with aqueous aluminum sulfate. Properties of these silicaproducts are listed in Table 14. The materials produced in theseexamples had high sphericity values and were well rounded in nature.Materials such as these may have applicability in paint and coatings andpaper applications.

Various modifications and variations can be made to the compounds,composites, kits, articles, devices, compositions, and methods describedherein. Other aspects of the compounds, composites, kits, articles,devices, compositions, and methods described herein will be apparentfrom consideration of the specification and practice of the compounds,composites, kits, articles, devices, compositions, and methods disclosedherein. It is intended that the specification and examples be consideredas exemplary.

What is claimed is:
 1. A continuous process for preparing a silica product, comprising: (a) continuously feeding an acidulating agent and an alkali metal silicate into a loop reaction zone comprising a stream of liquid medium; wherein at least a portion of the acidulating agent and the alkali metal silicate react to form a silica product in the liquid medium of the loop reaction zone; (b) continuously recirculating the liquid medium through the loop reaction zone; and (c) continuously discharging from the loop reaction zone a portion of the liquid medium comprising the silica product; wherein the loop reaction zone comprises a continuous loop of one or more loop reactor pipes.
 2. The process of claim 1, wherein the process further comprises controlling a temperature of the liquid medium with a heat exchanger in thermal communication with at least a portion of the one or more loop reactor pipes.
 3. The process of claim 1, wherein the acidulating agent and the alkali metal silicate are fed into the loop reaction zone at different points along the loop reaction zone.
 4. The process of claim 1, wherein the portion of the liquid medium discharged from the loop reaction zone is discharged in a volumetric rate proportional to the amount of the acidulating agent and the alkali metal silicate fed into the loop reaction zone.
 5. The process of claim 1, wherein the portion of the liquid medium discharged from the loop reaction zone is discharged in a volumetric rate equal to the amount of the acidulating agent and the alkali metal silicate fed into the loop reaction zone.
 6. The process of claim 1, wherein the process is carried out in a continuous single loop reactor.
 7. The process of claim 1, wherein the liquid medium is recirculated through the loop reaction zone at a rate in a range from 15 L/min to 100 L/min.
 8. A continuous process for preparing a silica product, comprising: (a) continuously feeding an acidulating agent and an alkali metal silicate into a loop reaction zone comprising a stream of liquid medium; wherein at least a portion of the acidulating agent and the alkali metal silicate react to form a silica product in the liquid medium of the loop reaction zone; (b) continuously recirculating the liquid medium through the loop reaction zone; and (c) continuously discharging from the loop reaction zone a portion of the liquid medium comprising the silica product; wherein the liquid medium is recirculated through the loop reaction zone at both: (1) a pH in a range from 2.5 to 10, and (2) a rate of from 97 vol. % to 645 vol. % per minute, based on the volume of the liquid medium in the loop reaction zone.
 9. The process of claim 8, wherein the pH is in a range from 6 to
 10. 10. The process of claim 8, wherein the pH is in a range from 7 to
 10. 11. The process of claim 8, wherein: the alkali metal silicate comprises sodium silicate; and the acidulating agent comprises sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, or combinations thereof.
 12. The process of claim 8, wherein: the alkali metal silicate comprises sodium silicate; and the acidulating agent comprises an acidic solution of aluminum sulfate.
 13. The process of claim 8, wherein the portion of the liquid medium discharged from the loop reaction zone is discharged in a volumetric rate proportional to the amount of the acidulating agent and the alkali metal silicate fed into the loop reaction zone.
 14. The process of claim 8, wherein the liquid medium comprising the silica product is recirculated through the loop reaction zone from 10 to 200 times prior to being discharged from the loop reaction zone.
 15. A continuous process for preparing a silica product, comprising: (a) continuously feeding an acidulating agent and an alkali metal silicate into a loop reaction zone comprising a stream of liquid medium; wherein at least a portion of the acidulating agent and the alkali metal silicate react to form a silica product in the liquid medium of the loop reaction zone; (b) continuously recirculating all of the liquid medium through the loop reaction zone; and (c) continuously discharging from the loop reaction zone a portion of the liquid medium comprising the silica product.
 16. The process of claim 15, wherein steps (a)-(c) are carried out simultaneously.
 17. The process of claim 15, wherein the liquid medium is recirculated through the loop reaction zone at a rate of from 97 vol. % to 645 vol. % per minute, based on the volume of the liquid medium in the loop reaction zone.
 18. The process of claim 15, wherein the liquid medium comprising the silica product is recirculated through the loop reaction zone from 10 to 200 times prior to being discharged from the loop reaction zone.
 19. The process of claim 15, wherein a pump is utilized to recirculate the liquid medium through the loop reaction zone.
 20. The process of claim 15, wherein the silica product comprises silica or sodium aluminosilicate.
 21. The method of claim 1, wherein the mean number of times the liquid medium is recirculated through the loop reaction zone is from 10 to
 200. 